NFC: Add an AAL Concept, too

While here, pull out some constants to their own header.  Eventually we'll
want to match on AalFeatures in the AAL Concept.

src/aal/aal.h

@@ -1,5 +1,8 @@
 #pragma once
+#include "../ds/concept.h"
 #include "../ds/defines.h"
+#include "aal_concept.h"
+#include "aal_consts.h"
 
 #include <chrono>
 #include <cstdint>
@@ -29,38 +32,6 @@
 
 namespace snmalloc
 {
-  /**
-   * Flags in a bitfield of attributes of this architecture, much like
-   * PalFeatures.
-   */
-  enum AalFeatures : uint64_t
-  {
-    /**
-     * This architecture does not discriminate between integers and pointers,
-     * and so may use bit operations on pointer values.
-     */
-    IntegerPointers = (1 << 0),
-    /**
-     * This architecture cannot access cpu cycles counters.
-     */
-    NoCpuCycleCounters = (1 << 1),
-    /**
-     * This architecture enforces strict pointer provenance; we bound the
-     * pointers given out on malloc() and friends and must, therefore retain
-     * internal high-privilege pointers for recycling memory on free().
-     */
-    StrictProvenance = (1 << 2),
-  };
-
-  enum AalName : int
-  {
-    ARM,
-    PowerPC,
-    X86,
-    X86_SGX,
-    Sparc,
-  };
-
   /**
    * Architecture Abstraction Layer. Includes default implementations of some
    * functions using compiler builtins.  Falls back to the definitions in the
@@ -160,7 +131,7 @@ namespace snmalloc
 {
   using Aal = AAL_Generic<AAL_Arch>;
 
-  template<AalFeatures F, typename AAL = Aal>
+  template<AalFeatures F, SNMALLOC_CONCEPT(ConceptAAL) AAL = Aal>
   constexpr static bool aal_supports = (AAL::aal_features & F) == F;
 } // namespace snmalloc
 

src/aal/aal_concept.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#ifdef __cpp_concepts
+#  include "../ds/concept.h"
+#  include "aal_consts.h"
+
+#  include <cstdint>
+#  include <utility>
+
+namespace snmalloc
+{
+  /**
+   * AALs must advertise the bit vector of supported features, their name,
+   * 
+   */
+  template<typename AAL>
+  concept ConceptAAL_static_members = requires()
+  {
+    typename std::integral_constant<uint64_t, AAL::aal_features>;
+    typename std::integral_constant<int, AAL::aal_name>;
+  };
+
+  /**
+   * AALs provide a prefetch operation.
+   */
+  template<typename AAL>
+  concept ConceptAAL_prefetch = requires(void *ptr)
+  {
+    { AAL::prefetch(ptr) } noexcept -> ConceptSame<void>;
+  };
+
+  /**
+   * AALs provide a notion of high-precision timing.
+   */
+  template<typename AAL>
+  concept ConceptAAL_tick = requires()
+  {
+    { AAL::tick() } noexcept -> ConceptSame<uint64_t>;
+  };
+
+  template<typename AAL>
+  concept ConceptAAL =
+    ConceptAAL_static_members<AAL> &&
+    ConceptAAL_prefetch<AAL> &&
+    ConceptAAL_tick<AAL>;
+
+} // namespace snmalloc
+#endif

src/aal/aal_consts.h

@@ -0,0 +1,37 @@
+#pragma once
+#include <cstdint>
+
+namespace snmalloc
+{
+  /**
+   * Flags in a bitfield of attributes of this architecture, much like
+   * PalFeatures.
+   */
+  enum AalFeatures : uint64_t
+  {
+    /**
+     * This architecture does not discriminate between integers and pointers,
+     * and so may use bit operations on pointer values.
+     */
+    IntegerPointers = (1 << 0),
+    /**
+     * This architecture cannot access cpu cycles counters.
+     */
+    NoCpuCycleCounters = (1 << 1),
+    /**
+     * This architecture enforces strict pointer provenance; we bound the
+     * pointers given out on malloc() and friends and must, therefore retain
+     * internal high-privilege pointers for recycling memory on free().
+     */
+    StrictProvenance = (1 << 2),
+  };
+
+  enum AalName : int
+  {
+    ARM,
+    PowerPC,
+    X86,
+    X86_SGX,
+    Sparc,
+  };
+} // namespace snmalloc

src/ds/concept.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <type_traits>
+
 /**
  * C++20 concepts are referenced as if they were types in declarations within
  * template parameters (e.g. "template<FooConcept Foo> ...").  That is, they